While the patent literature on the roasting of metal sulphide concentrates is extensive, the bulk of the art relates to methods for roasting at temperatures insufficient to cause fusion and the resulting agglomeration of the particles. A select few patents relate to fluid bed roasting at temperatures sufficiently high to cause agglomeration, and even fewer teach the feeding of sulphides to fluid bed roasters as aqueous slurries rather than as discrete particles or particle agglomerates.
While the present invention is an improved method for agglomerative roasting and a detailed review of non-agglomerative roasting is therefore unnecessary, brief comment is in order to draw attention to the characteristic disadvantages of such roasting in general and the resulting incentives to develop successful means for agglomerative roasting. A reading of U.S. Pat. Nos. 2,556,215; 2,785,050; 2,943,929; 2,993,778; and 3,160,496 provides a reasonable indication of the breadth of methods for non-agglomerative roasting and their principal characterizing features. Thus it is seen that in general these patents teach fluid bed roasting at temperatures below 1000.degree.C with dry sulphide feeds and the characteristically disadvantageous results of inefficient and incomplete roasting, high air factors, fine product, high recirculating loads and roaster gas heavily laden with dust, free oxygen and SO.sub.3.
Such disadvantages should be at least partly overcome by roasting at higher temperature to take advantage of faster reaction rates and agglomeration of particles resulting from fusion of sulphides. The fact is, however, that existing methods for agglomerative fluid bed roasting of sulphides are fraught with their own difficulties and disadvantages, as indicated by a reading of U.S. Pat. Nos. 2,796,340; 2,819,157; 2,850,370 and 3,094,409. These methods teach feeding of metal sulphides to the bed as discrete particles or particle agglomerates and roasting at temperatures generally greater than about 1000.degree.C. Roasting is either incomplete or achieved only by adoption of expedients such as oxygen-enriched air and high air factors, thereby leading to roaster off-gas containing excess free oxygen. Difficulty is also experienced in preventing defluidization resulting from excessive agglomerative growth of bed particles as indicated by the adoption of various other undesirable practices. In U.S. Pat. No. 2,850,370, for example, the temperature of the bed is cycled above and below the temperature of incipient sintering of particles, apparently in an attempt to control agglomerative growth of the bed particles. The need for such an expedient indicates inability to control agglomeration at the upper end of the temperature range thereby requiring the temperature to be lowered periodically to decrease or stop particle growth. The effect of the lower temperature is to decrease not only agglomeration, however, but also the rate and therefore the degree of extent of roasting, and also to increase the generation of fine particles that become entrained in the roaster off-gas, all undesirable side effects of this temperature cycling technique.
Prior to the development of the present invention our own attempts to effect agglomerative fluid bed roasting at temperatures high enough to cause fusion were similarly unsuccessful as the other such methods referred to above. Thus we found that when finely divided pyrrhotite particles were fed dry to a fluid bed of roasted calcine particles, there was incomplete sulphur elimination at bed temperatures below about 1000.degree. C, while above this temperature there was uncontrolled, catastrophic growth of the calcine particles that eventually resulted in defluidization. Similar effects resulted from dry feeding of a chalcopyrite concentrate to a fluid bed roasting operation at somewhat lower temperatures around about 950.degree.C.
Two patents relating to the feeding of sulphides to fluid bed roasters as aqueous slurries are relevant to the present discussion. One is U.S. Pat. No. 2,677,608 which teaches that in the conduct of exothermic processes in fluid beds, such as roasting of sulphides, the feed solids should be charged as an aqueous slurry through the sidewalls of the fluid bed reactor and into the fluid bed, not only to simplify the feeding of the solids themselves but also to provide water to the bed as a cooling medium, and specifically to prevent the temperature of the bed rising during the progress of the exothermic process to the point that fusion occurs, at which temperature defluidization has been the anticipated, and commonly the inevitable result.
The other patent is U.S. Pat. No. 2,813,015 which teaches feeding sulphides to a fluid bed roaster as an aqueous slurry through the freeboard together with compressed air to disperse the slurry into a spray of slurry droplets from which water evaporates during their descent through the freeboard to form first wet agglomerates and finally dry agglomerates which enter the bed where they are roasted. The desirability of evaporating slurry water in the freeboard and not in the bed is repeatedly emphasized as a cornerstone of the method. The sulphide agglomerates are held together by a binder, advantageously sodium sulphate for the preferred practice of the invention to sulphatize nickel selectively with respect to iron in sulphide concentrates. Such sulphation is conducted at temperatures at or below about 700.degree.C and generates roaster gas that contains not only SO.sub.3 and free oxygen but also much finely divided dust.
Apart from the disadvantages of the existing methods reviewed above, a major weakness that is common to all of them is the generation of roaster gas with undesirable constituents for subsequent treatment to recover sulphur, particularly by reduction of SO.sub.2 in the gas to elemental sulphur. Thus in general, the gases contain either or both free oxygen, which will consume fuel in the reduction process, and dust that is difficult to disengage even in electrostatic precipitators never mind in cyclones. In addition, those gases produced by the lower temperature methods also contain SO.sub.3.
In summary then, there are no existing methods, of which we are aware, by which metal sulphides can be agglomeratively roasted in a fluid bed to produce advantageously not only a substantially dead-roasted, sulphur-free calcine on the one hand, but also a roaster gas on the other hand from which entrained solid particles can be readily and substantially completely removed and which furthermore is substantially devoid not only of SO.sub.3 but also of free oxygen so that the gas can be readily treated for recovery of sulphur, particularly by reduction of SO.sub.2 in the gas to elemental sulphur. Such are the achievements of the present invention.
One particular advantage of the present method as applied specifically to nickeliferous pyrrhotite concentrates is the production of a calcine which, contrary to the teachings of the prior art, is readily treated for recovery of nickel therefrom by either of two existing methods -- the one comprising gaseous reduction of the nickel in the calcine followed by leaching thereof in ammoniacal solutions, and the other comprising sulphatization of the nickel in the calcine followed by leaching thereof in water.
Thus in reading Canadian Pat. Nos. 530,842; 593,622 and 607,302, one finds repeated statements to the effect that calcines resulting from the roasting of nickeliferous pyrrhotite concentrates are not responsive either to the reduction-ammonia leach method, if the roasting is done at temperatures in excess of about 1600.degree.F, i.e. about 870.degree.C, or to the sulphating-water leach method if the roasting temperature is in excess of about 1400.degree.F, i.e. about 760.degree.C. It is emphasized that the calcines should be unagglomerated and porous and that temperatures that would cause sintering, fusion, densification, and the like are to be specifically avoided, that is to say, for example, temperatures in excess of 1000.degree.C or so at which temperatures agglomerative roasting of pyrrhotite occurs. Thus the calcines generated by the methods described in the patents referred to above are finely divided with particle sizes characteristically less than about 200 Tyler mesh, the operations are dusty, and because roasting is effected with air in considerable excess of that stoichiometrically required to convert the pyrrhotite to hematite and SO.sub.2, the roaster off-gases contain significant concentrations of free oxygen as well as copious quantities of fine calcine dust particles. These and other disadvantages are overcome by the practice of the present invention and in addition, described and discussed in more detail below, calcine resulting from roasting nickeliferous pyrrhotite concentrates by this invention is readily responsive to both the methods for extraction of nickel referred to above.